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Although the Silurian age sediments were deposited on a relatively flat surface, impressive reefs were also forming at this time. These were built by corals and shelled marine animals, are almost pure carbonate, and, in general, contain of fossils as impressions and molds (Willman, 1971).
Whereas the bedrock at Midewin is exposed only in places where the surface has been eroded by streams or glacial meltwater, most features of the landscape owe their existence to the repeated advance and retreat of the ice associated with Wisconsin glaciation. There is geological evidence for previous glaciations in Illinois over the last 1.6 million years, but the last of these, the Wisconsin glaciation, is primarily responsible for the topography at the site. The Wisconsin ice advanced and retreated several times out of the Lake Michigan basin and into Illinois between 50,000 (Curry, et al. 1992) and 13,500 years ago (Hansel and Johnson, 1992). The advancing ice overrode the features and deposits left by earlier glaciations and, at its maximum, spread southwest almost to the Mississippi river and south into Central Illinois as far as Shelbyville (Willman, 1971). The Midewin site is located in a physiographic section of Illinois known as the Kankakee Till Plain-a level to gently rolling plain of dolomitic bedrock covered by glacial deposits (called drift) of varying thickness (Berg, 1987). The relatively flat topography of till plains in Illinois is a major factor in the development of prairie in the region.
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